JP4409407B2 - Magnetite particles and method for producing the same - Google Patents

Description

The present invention relates to a luma Gunetaito particles to contain calcium and phosphorus.

  The present applicant has previously proposed magnetite particles containing calcium (see Patent Documents 1 and 2). In patent document 1, the magnetite particle | grains in which the composite iron oxide layer containing aluminum and calcium was formed in the surface are disclosed. The magnetite particles have high electrical resistance and high saturation magnetization, and are particularly suitable as electrostatic powder magnetic carrier material powder and magnetic toner material powder. On the other hand, the technique described in Patent Document 2 attempts to improve the mixing property with a resin by incorporating a light element such as calcium into the magnetite particles to reduce the specific gravity thereof. However, in Patent Document 1, although there is a study on the environmental stability of saturation magnetization, there is no particular study on highly dispersing particles while enhancing the environmental stability of the electrical resistance value. In Patent Document 2, the environmental stability of the electrical resistance value is not particularly examined. In addition, no study has been made on highly dispersing particles.

  Apart from these, magnetite particles containing phosphorus have been proposed (Patent Documents 3 and 4). The techniques described in these patent documents attempt to improve the dispersibility of particles by containing phosphorous to lower the residual magnetization and prevent magnetic aggregation. However, even in these patent documents, the environmental stability of the electrical resistance value is not particularly studied.

JP 2003-267331 A JP 2000-335920 A JP-A-8-169717 Japanese Patent Laid-Open No. 10-101339

Accordingly, an object of the present invention is to provide magnetite particles having high particle dispersibility while enhancing the environmental stability of the electrical resistance value.

This invention achieves the said objective by providing the magnetite particle | grains characterized by the particle | grain surface being coat | covered with the composite iron oxide layer containing calcium and phosphorus.

Further, the present invention provides a preferred method for producing the magnetite particles as follows:
To the slurry of magnetite core particles in the presence of calcium ions, a ferrous salt aqueous solution containing a water-soluble phosphorus compound and an alkaline aqueous solution are added and mixed, and wet oxidation is performed while adjusting the pH to 8-13. The present invention provides a method for producing magnetite particles, wherein a composite iron oxide layer containing calcium and phosphorus is formed on the particle surface.

Further, the present invention provides another preferred method for producing the magnetite particles as follows:
A ferrous salt aqueous solution and an alkaline aqueous solution are added to and mixed with a slurry of magnetite core particles in the presence of calcium ions, and wet oxidation is performed while adjusting the pH to 8 to 13. The present invention provides a method for producing magnetite particles, which is performed while adding a phosphorus compound.

The magnetite particles of the present invention have good environmental stability in terms of electrical resistance and are excellent in particle dispersibility. Therefore, the magnetite particles of the present invention are particularly preferably used as a black pigment powder for paint or a material powder for electrostatic copying magnetic toner.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The iron oxide particles referred to in the present invention are preferably composed mainly of magnetite. In the following description, magnetite particles that are typical examples of iron oxide particles will be described. The term “iron oxide particles” or “magnetite particles” means either individual particles or aggregates depending on the content.

  The magnetite particles of the present invention are composed of core magnetite particles (hereinafter also referred to as core particles) and a composite iron oxide layer covering the surface of the core particles. As will be described later, the core particles are preferably produced by a wet method, but may be produced by a dry method. The core particles may contain appropriate amounts of various effective elements such as silicon and aluminum. The composite iron oxide layer covering the core particles contains calcium, phosphorus, iron and oxygen. It is considered that calcium and phosphorus are present in the composite iron oxide layer, for example, in the state of a compound of phosphorus and calcium or in the state of a composite oxide with iron.

  Calcium may be present only in the composite iron oxide layer, or may be present from the center to the surface of the magnetite particles. Preferably, it exists in both the inside of a particle | grain and a composite iron oxide layer, and is unevenly distributed in a composite iron oxide layer. This produces an advantageous effect that the electrical resistance of the particles can be increased appropriately. From this viewpoint, the amount of calcium present in the composite iron oxide layer is preferably 0.01 to 0.9% by mass, particularly 0.05 to 0.8% by mass, based on the mass of the entire particle. The fact that calcium is unevenly distributed means that the molar ratio of Ca / Fe in the composite iron oxide layer is higher than the molar ratio of Ca / Fe in the core particles.

  It is preferable that phosphorus is present only in the composite iron oxide layer and is not substantially present in the core particle. That is, it is preferable that the ratio of the amount of phosphorus present in the composite iron oxide layer to the mass of the entire particle is substantially the same as the ratio of the amount of phosphorus contained in the entire particle to the mass of the entire particle. This has the advantageous effect of improving the dispersibility of the particles. The ratio of the amount of phosphorus contained in the entire particle to the mass of the entire particle will be described later.

  The amount of each of calcium and phosphorus present in the composite iron oxide layer with respect to the whole particle is measured by the following method. First, the magnetite particles before the formation of the composite iron oxide layer and the magnetite particles after the formation of the composite iron oxide layer corresponding to the finally produced particles 20 g are completely dissolved in the acid separately. Each solution is quantified by ICP, and the mass difference is defined as the elemental amount of the composite iron oxide layer and the outer portion of the particle. Let this amount be A. On the other hand, 20 g of magnetite particle powder after forming the composite iron oxide layer is suspended in 100 ml of 0.01 mol / l potassium hydrogen phthalate aqueous solution (pH of 4.01 at 25 ° C.) and stirred at 25 ° C. for 3 hours. . After stirring, the solution is filtered through a membrane filter having a pore size of 0.1 μm, and the amounts of calcium and phosphorus in the obtained filtrate are analyzed by ICP. The element amount of the outer part from the layer) is determined. Let this amount be B. From the above results, the content (%) of each element in the composite iron oxide layer is determined by (AB) / 20 × 100.

  The amount of calcium contained in the entire particle is preferably 0.05 to 1% by mass, particularly 0.1 to 0.9% by mass, based on the mass of the entire particle, from the viewpoint of magnetic properties. On the other hand, the amount of phosphorus contained in the whole particle is 0.05 to 1% by mass, particularly 0.1 to 0.9% by mass, based on the mass of the whole particle. It is preferable from the viewpoint of balance.

  The amount of each of calcium and phosphorus in the whole particle with respect to the whole particle can be measured by ICP using a solution obtained by completely dissolving the particle in an acidic solution as a sample.

  There is no restriction | limiting in particular in the particle shape of the magnetite particle | grains of this invention, For example, it can be set as the octahedron shape, spherical shape, hexahedron shape, and polyhedron shape more than an octahedron. As for the particle size of the magnetite particles, it is preferable that the average particle size of the primary particles is about 0.15 to 0.3 μm from the viewpoint that the resolution can be improved when the magnetite particles are used as, for example, a magnetic toner material powder. Magnetic toners tend to have a smaller particle size for improved resolution, and for such toners, it is preferable to reduce the particle size of the magnetite used. However, magnetite particles having a reduced particle size tend to lack stability in various properties. On the other hand, the magnetite particles of the present invention have a particularly excellent environmental stability while having a small particle size. The average particle diameter of the primary particles is obtained by observing the sample with a TEM and using the ferret diameter.

  In general, magnetite particles having high remanent magnetization, particularly octahedral magnetite particles, deteriorate in dispersibility due to magnetic aggregation. On the other hand, the magnetite particles of the present invention have high dispersibility even if the residual magnetization is high (the residual magnetization will be described later). Specifically, the value of the specular reflectance (incident angle 60 °) serving as a dispersibility index is as high as 60 to 100%, particularly 70 to 100%. The present inventors believe that the reason is that the surface of the particle is covered with the composite iron oxide layer containing calcium and phosphorus described above.

  The specular reflectance is measured by the following method. A liquid in which a styrene acrylic resin (TB-1000F) is dissolved in toluene (resin: toluene = 1: 2) is prepared. In addition, glass beads having a diameter of 1 mm are prepared. 60 g of this liquid, 90 g of glass beads, and 10 g of magnetite particles are put into a bottle having an internal volume of 140 ml and capped. The contents of the bottle are mixed with a paint shaker (manufactured by Toyo Seiki Co., Ltd.) for 30 minutes. The mixed contents are applied onto a glass plate using a 4 mil applicator and dried to obtain a coating film. The specular reflectance of this coating film is measured using a Murakami type GLOSS METER (GM-3M). The incident angle is 60 °.

Due to the surface of the particles being coated with a composite iron oxide layer containing calcium and phosphorus, the magnetite particles of the present invention have excellent environmental stability. The present inventors consider that this is because the increase in the specific surface area of the particles is suppressed by the presence of the composite iron oxide layer. In this connection, the magnetite particles of the present invention have a low specific surface area by the BET method of preferably 4 to 15 m ² / g, more preferably 5 to 14 m ² / g. The specific surface area is measured by, for example, a 2200 type BET meter manufactured by Shimadzu-Micromeritics.

As a measure of environmental stability, the present invention adopts the ratio of volume electric resistance of magnetite particles after being exposed for 24 hours in each environment of 10 ° C./20% RH and 35 ° C./85% RH. Yes. Specifically, the value of the volume electrical resistance after 24 hours exposure in a 10 ° C./20% RH environment is R _LL, and the volume electrical resistance after 24 hours exposure in a 35 ° C./85% RH environment. when the value was defined as R _HH, the value of R _HH / R _LL (hereinafter this that the value and volume resistivity ratio) is preferably as close to 1. The lower limit is 0.3, particularly 0.4, and particularly preferably 0.5. In general, it is known that the value of volume resistivity decreases as the humidity increases. However, the magnetite particles of the present invention have little change in electrical resistance due to differences in the environment, and are stable with respect to differences in the environment. Has characteristics. As a result, when the magnetite particles of the present invention are used as a magnetic toner material powder, they can be stably transferred to a photosensitive drum under various environments. That is, by using the magnetite particles of the present invention for the magnetic toner material powder, it can be stably transferred to the photosensitive drum without depending on various environments.

The value of the volume resistivity ratio, which is the ratio of R _HH to R _LL , is as described above, and the values of R _HH and R _LL itself are preferably as follows. That is, R _HH is preferably 1 × 10 ² to 1 × 10 ⁶ Ω · cm, and more preferably 1 × 10 ³ to 1 × 10 ⁵ Ω · cm from the viewpoint of electrical characteristics when used as a toner material powder. . R _LL is preferably 1 × 10 ² to 1 × 10 ⁶ Ω · cm, and more preferably 1 × 10 ³ to 1 × 10 ⁵ Ω · cm, for the same reason.

The value of the volume electrical resistance is measured by the following method. Magnetite particles are exposed in an environmental chamber at 10 ° C./20% RH and 35 ° C./85% RH for 24 hours. Next, 10 g of magnetite particles are placed in a holder, and a pressure of 600 kg / cm ² is applied to form a 25 mmφ tablet. An electrode is attached to the molded body, and the electric resistance is measured under a pressure of 150 kg / cm ² . The volume resistance value is obtained from the thickness and cross-sectional area of the sample used for the measurement and the resistance value.

Magnetite particles of the present invention, the saturation magnetization σs under conditions multiplied by the external magnetic field of 796 kA / m is 70~92Am ² / kg, in particular 75~90Am ² / kg, especially 80~90Am ² / kg From the viewpoint of preventing image deterioration due to poor frictional chargeability of the magnetic toner. From the same point of view, it is preferable that the residual magnetization σr under the condition of applying an external magnetic field of 796 kA / m is 3 to 25 Am ² / kg, particularly 5 to ²⁰ Am ² / kg. Furthermore, it is preferable that the coercive force under a condition where an external magnetic field of 796 kA / m is applied is 3 to 25 kA / m, particularly 5 to 20 kA / m. These magnetic properties are measured using, for example, a vibrating sample magnetometer VSM-P7 manufactured by Toei Kogyo.

  The magnetite particles of the present invention preferably have an FeO content of 18% by mass or more from the viewpoint of obtaining sufficient magnetic properties and increasing blackness. The FeO content in the magnetite particles affects the blackness of the magnetite particles. From the viewpoint of environmental stability, it is preferable that the FeO deterioration rate is low. Specifically, the FeO deterioration rate after exposure of magnetite particles at 150 ° C. for 1 hour in the air is preferably 10% or less. In this connection, the blackness of the magnetite particles of the present invention preferably has an L value of 18.5 or less. The blackness is measured using a color difference meter with the coating film prepared in the above-described measurement of the specular reflectance as a measurement target. The FeO deterioration rate is calculated from the FeO content before and after putting magnetite particles in a watch glass, holding it at 150 ° C. for 1 hour in a ventilating dryer (Oven PH-201 manufactured by Tabai Espec). The FeO content is measured by redox titration using magnetite particles dissolved in sulfuric acid and using a potassium permanganate standard solution.

  Next, the preferable manufacturing method of the magnetite particle | grains of this invention is demonstrated. This production method comprises a first step of preparing a slurry of iron oxide core particles in the presence of calcium ions, and a ferrous salt aqueous solution containing a water-soluble phosphorus compound in the slurry obtained in the first step. And a second step of forming a composite iron oxide layer containing calcium and phosphorus on the surface of the core particles by adding and mixing an alkaline aqueous solution and performing wet oxidation while adjusting the pH to 8 to 13. Hereinafter, each step will be described.

  In the first step, to prepare a slurry of iron oxide core particles in the presence of calcium ions, for example, under conditions where calcium ions coexist in the ferrous hydroxide slurry, A method of wet oxidation of the ferrous slurry can be used. In addition, since calcium ion has a large ionic radius, it has a feature that it is difficult to be taken into the core particle.

  Calcium ions can be present in the system before the production of ferrous hydroxide. For example, when obtaining a ferrous hydroxide slurry by neutralizing and mixing an alkaline aqueous solution and a ferrous salt aqueous solution, either one of the alkaline aqueous solution and the ferrous salt aqueous solution contains a water-soluble calcium compound, Neutralization and mixing can produce ferrous hydroxide. In this case, the water-soluble calcium compound is preferably contained in the alkaline aqueous solution rather than the ferrous salt aqueous solution.

  Examples of the water-soluble calcium compound include calcium hydroxide, calcium carbonate, calcium chloride and the like. As the alkali, for example, sodium hydroxide, potassium hydroxide, sodium carbonate or the like is used. On the other hand, as the ferrous salt, for example, ferrous sulfate, ferrous chloride and the like are used. The amount of alkali used is appropriately selected according to the shape of the target magnetite particles. For example, when obtaining spherical magnetite particles, the amount of alkali used is preferably a neutralization equivalent of ferrous ions. When obtaining octahedral magnetite particles, the amount of alkali used is preferably at least the neutralization equivalent of ferrous ions. The amount of water-soluble calcium compound used is 0 in terms of a molar ratio (calcium ion usage (mol) / ferrous ion usage (mol) × 100 (%)) with respect to 1 mol of ferrous ion. 0.1 to 3.0%, particularly 0.3 to 2.8% is preferable. By making the amount used within this range, a sufficient amount of calcium ions will remain in the slurry at the end of the first stage reaction.

  Wet oxidation is performed by bubbling an oxygen-containing gas, typically air, through the ferrous hydroxide slurry. The slurry temperature during oxidation is preferably about 70 to 98 ° C. Magnetite core particles are obtained by wet oxidation. As described above, since calcium ions are hardly taken into the core particles, the content of calcium in the obtained core particles is not so high. Therefore, calcium ions remain in the slurry after the core particles are generated. If the remaining amount of calcium ions is not sufficient, a water-soluble calcium compound can be additionally added at an appropriate time as described later.

  As described above, the particle shape of the magnetite particles of the present invention is not particularly limited. However, when the pH in the first step is a neutral region of less than 8, spherical magnetite particles are obtained, and 8 to 10.5. In this case, hexahedral magnetite particles are obtained, and when it exceeds 10.5, octahedral magnetite particles are obtained. Accordingly, if any one of the conditions is selected, the target shape can be obtained.

  The wet oxidation is preferably carried out until no unreacted iron ions remain in the slurry. After confirming that unreacted iron ions do not remain, the aeration of the oxygen-containing gas is stopped and the first step is completed. However, wet oxidation may be stopped in a state where a certain amount of iron ions remains, and the first step may be terminated.

  In the first step, calcium ions can be added later to the slurry of magnetite core particles obtained by wet oxidation of the ferrous hydroxide slurry. Specifically, the ferrous hydroxide slurry is wet-oxidized according to a conventional method to produce a slurry of magnetite core particles. Next, a water-soluble calcium compound is separately added to the slurry. According to this method, the concentration of calcium ions in the slurry of magnetite core particles can be accurately controlled.

  As a result of the examination by the present inventors, the concentration of calcium ions in the slurry of the magnetite core particles affects the performance of the obtained magnetite particles. Specifically, the calcium ions present in the magnetite core particle slurry preferably have a low concentration. This is because if the calcium ion concentration is too high, calcium and a water-soluble phosphorus compound react with each other in the second step to be described later, and a water-insoluble or poorly water-soluble product tends to be generated in the slurry. From this point of view, regardless of whether calcium ions are added before the production of magnetite core particles or added separately after the production, the concentration of calcium ions is 0.001 to 0.05 mol / l, particularly 0. It is preferable to set it to 002-0.04 mol / l.

  When a slurry of magnetite core particles in the presence of calcium ions is prepared in the first step, the second step is performed. In the second step, first, an aqueous ferrous salt solution and an aqueous alkaline solution containing a water-soluble phosphorus compound are added to and mixed with the slurry obtained in the first step. As the water-soluble phosphorus compound, for example, phosphates such as sodium phosphate, potassium phosphate and ammonium phosphate, orthophosphoric acid, phosphorous acid and the like are used. The amount of the water-soluble phosphorus compound used is 0.4 in terms of the molar ratio of phosphorus to 1 mol of ferrous ions (phosphorus usage (mol) / ferrous ion usage (mol) × 100 (%)). It is preferable to be 10%, particularly 0.8% to 8%.

  An aqueous ferrous salt solution and an aqueous alkaline solution containing a water-soluble phosphorus compound are added to and mixed with the slurry obtained in the first step, and wet oxidation is performed. If necessary, an appropriate amount of a water-soluble calcium compound may be added after the addition of an aqueous ferrous salt solution and an aqueous alkaline solution containing a water-soluble phosphorus compound. The amount added is preferably such that the concentration of calcium ions in the slurry is in the range described above. It is important to adjust the pH of the wet oxidation to 8 to 13, particularly 9 to 12. As a result, the calcium ions present in the slurry react with the water-soluble phosphorus compound to produce a water-insoluble or poorly water-soluble reaction product, and the composite iron oxide layer forms the surface of the core particle while the reaction product is taken in. Formed. For example, when phosphate or orthophosphoric acid is used as the water-soluble phosphorus compound, calcium phosphate which is a poorly water-soluble substance is generated, and a complex iron oxide layer is formed while it is taken in. As a result, calcium and phosphorus are unevenly distributed in the composite iron oxide layer. Due to this uneven distribution, an increase in the specific surface area of the magnetite particles is suppressed, and the environmental stability is improved.

  Adjustment of pH in the second stage reaction is performed by adding an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide. The wet oxidation conditions in the second step can be the same as those in the first step. The wet oxidation is performed until no unreacted iron ions remain in the slurry. After the completion of the second step, the reaction product is filtered, washed, dried and pulverized to obtain the magnetite particles of the present invention.

  As an alternative to the second step, a ferrous salt aqueous solution and an alkaline aqueous solution are added to and mixed with a slurry of magnetite core particles in the presence of calcium ions, and wet oxidation is performed while adjusting the pH to 8-13. And a method of performing the wet oxidation while adding a water-soluble phosphorus compound. The water-soluble phosphorus compound is preferably added continuously and gradually to the reaction system in the form of an aqueous solution. The amount of the water-soluble phosphorus compound used is 0.4 in terms of the molar ratio of phosphorus to 1 mol of ferrous ions (phosphorus usage (mol) / ferrous ion usage (mol) × 100 (%)). It is preferable to be 10%, particularly 0.8% to 8%. The concentration of the water-soluble phosphorus compound in the aqueous solution of the water-soluble phosphorus compound is preferably 0.1 to 1.0 mol / l, particularly preferably 0.2 to 0.5 mol / l. Also in this method, the calcium ions present in the slurry react with the water-soluble phosphorus compound to produce a water-insoluble or poorly water-soluble reaction product, and the composite iron oxide layer becomes the core particle while the reaction product is taken in. Formed on the surface.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples.

[Example 1]
29 liters of an aqueous solution containing 0.045 mol / l calcium hydroxide and 5 mol / l sodium hydroxide and 35 liters of ferrous sulfate aqueous solution containing 2 mol / l Fe ²⁺ were mixed and stirred, and ferrous hydroxide A slurry containing was obtained. The pH at this time was 12. While maintaining this slurry at 85 ° C., air was blown at 65 l / min to perform wet oxidation for 300 minutes to obtain core particles.

After Fe ²⁺ ions unreacted in the slurry and it was confirmed that no remaining to complete the first step to stop the blowing of air. Next, 15 liters of ferrous sulfate aqueous solution containing 0.085 mol / l orthophosphoric acid and 2 mol / l of Fe ²⁺ was added to and mixed with the slurry, and air was blown again at 65 l / min to perform wet oxidation for 120 minutes. In the wet oxidation process, an aqueous sodium hydroxide solution was appropriately added to the slurry to maintain the pH of the slurry at 10. After confirming that unreacted Fe ²⁺ ions did not remain in the slurry, air blowing was stopped and the second step was completed. The obtained particles were filtered, washed, dried and pulverized to obtain magnetite particles.

[Example 2 and Comparative Examples 1 and 2]
Magnetite particles were obtained in the same manner as in Example 1 except that the amounts of raw materials charged in the first step and the second step were set to the values shown in Tables 1 and 2.

Example 3
29 liters of an aqueous solution containing 5 mol / l sodium hydroxide and 35 liters of a ferrous sulfate aqueous solution containing 2 mol / l of Fe ²⁺ were mixed and stirred to obtain a slurry containing ferrous hydroxide. The pH at this time was 12. While maintaining this slurry at 85 ° C., air was blown at 65 l / min to perform wet oxidation to obtain core particles. After confirming that no unreacted Fe ²⁺ ions remained in the slurry, the blowing of air was stopped. To this slurry, 1.1 liter of an aqueous solution containing 1 mol / l calcium hydroxide was added, and the first step was completed.

Next, in the second step, 15 liters of ferrous sulfate aqueous solution containing 0.085 mol / l orthophosphoric acid and 2 mol / l of Fe ²⁺ is added to and mixed with the slurry, and air is blown again at 65 l / min to perform wet oxidation again. It was. In the wet oxidation process, an aqueous sodium hydroxide solution was appropriately added to the slurry to maintain the pH of the slurry at 10. After confirming that unreacted Fe ²⁺ ions did not remain in the slurry, air blowing was stopped and the second step was completed. The obtained particles were filtered, washed, dried and pulverized to obtain magnetite particles.

Example 4
The first step was the same as in Example 3 . Next, in the second step, 15 liters of a ferrous sulfate aqueous solution containing 2 mol / l of Fe ²⁺ was added to and mixed with the slurry, and air was blown at 65 l / min to perform wet oxidation again. In the wet oxidation process, an aqueous sodium hydroxide solution was appropriately added to the slurry to maintain the pH of the slurry at 10. In addition, 1.1 liter of 1 mol / l orthophosphoric acid was continuously and gradually added to the slurry. After confirming that unreacted Fe ²⁺ ions did not remain in the slurry, air blowing was stopped and the second step was completed. The obtained particles were filtered, washed, dried and pulverized to obtain magnetite particles.

[Performance evaluation]
The magnetite particles obtained in Examples and Comparative Examples were subjected to chemical analysis and various properties were measured by the methods described above. These results are shown in Tables 3 and 4.

As is clear from the results shown in Tables 1 to 4, the magnetite particles of the respective examples (product of the present invention) have higher specular reflectivity and excellent dispersibility than the magnetite particles of the respective comparative examples. I understand that. Furthermore, it can be seen that the value of R _HH / R _LL is close to 1 and the electrical resistance value is excellent in environmental stability. It can also be seen that the value of saturation magnetization is sufficient with respect to the magnetic characteristics. On the other hand, in the magnetite particles of Comparative Example 1 that do not contain calcium, it can be seen that the environmental stability of the electrical resistance value is not good. It can be seen that the magnetite particles of Comparative Example 2 containing no phosphorus are not good in environmental stability of the electrical resistance value due to the large specific surface area, are low in specular reflectance, and are not good in dispersibility.

Claims (9)

A magnetite particle, wherein the particle surface is covered with a composite iron oxide layer containing calcium and phosphorus. The magnetite particle according to claim 1, wherein calcium is present both in the particle and in the composite iron oxide layer, and calcium is unevenly distributed in the composite iron oxide layer. The amount of calcium contained in the whole particle is 0.05 to 1% by mass with respect to the mass of the whole particle, and the amount of calcium present in the composite iron oxide layer is 0.01% with respect to the mass of the whole particle. The magnetite particle according to claim 2, which is ˜0.9 mass%. The magnetite particle according to any one of claims 1 to 3, wherein phosphorus is present only in the composite iron oxide layer, and the amount of phosphorus is 0.05 to 1% by mass relative to the mass of the whole particle. A method for producing magnetite particles according to claim 1,
To the slurry of magnetite core particles in the presence of calcium ions, a ferrous salt aqueous solution containing a water-soluble phosphorus compound and an alkaline aqueous solution are added and mixed, and wet oxidation is performed while adjusting the pH to 8-13. A method for producing magnetite particles, comprising forming a composite iron oxide layer containing calcium and phosphorus on the particle surface. A method for producing magnetite particles according to claim 1,
A ferrous salt aqueous solution and an alkaline aqueous solution are added to and mixed with a slurry of magnetite core particles in the presence of calcium ions, and wet oxidation is performed while adjusting the pH to 8 to 13. A method for producing magnetite particles, comprising adding a phosphorus compound. Slurry of magnetite core particles in a state in which calcium ions are present, the manufacture of claim 5 or 6 magnetite particles according ferrous hydroxide slurry containing a water-soluble calcium compounds are those generated by wet oxidation Method. When obtaining the ferrous hydroxide slurry containing the water-soluble calcium compound, the alkali aqueous solution or the ferrous salt aqueous solution contains the water-soluble calcium compound , and the alkali aqueous solution and the ferrous salt aqueous solution are neutralized. method for producing magnetite particles of the resulting Ru claim 7, wherein said aqueous ferrous oxide slurry by mixing. The method for producing magnetite particles according to any one of claims 5 to 8, wherein a water-soluble phosphorus compound that reacts with calcium to produce a water-insoluble or hardly water-soluble reaction product is used.